Project

Discipline

calcium homeostasis; calcium sensor; neurodegnerative disease; adult neurogenesis; calcium-binding proteins; neurological diseases; calcium homeostasome; compensation mechanisms; transgenic mice; behavior

Lead
Lay summary
Calcium ions (Ca2+) act as important second messengers. Ca2+ signaling is a key mechanism for diverse biological processes such as cell growth, differentiation and cell death; it operates on a huge range of time scales (from ms to hours) and magnitudes (from 10-9M to 10-3M). For this, cells make use of various proteins from the Ca2+ signaling toolkit comprising proteins involved in Ca2+ entry/extrusion from either the extracellular space or from intracellular compartments (ER, mitochondria) and specific Ca2+-binding proteins acting as intracellular modulators of Ca2+ signals. Each cell type expresses a particular set of components involved in Ca2+ handling/signaling. These molecules are not only functionally linked, but likely also regulated in a network-like fashion forming the Ca2+ homeostasome. In view of the large complexity of Ca2+ signaling, it is not surprising that the malfunctioning or disruption of intracellular Ca2+ homeostasis has been associated with ageing, neurodegeneration and various diseases that includes epileptic seizures, migraine, bipolar disorder and autism. We are interested in the physiological role of specific Ca2+-binding proteins (CaBPs) including parvalbumin (PV), calbindin D-28k (CB) and calretinin (CR). While initially thought to act as simple Ca2+ buffers, newer experimental work using transgenic mice demonstrated that these CaBPs are key factors for the modulation of cellular Ca2+ dynamics. How a given CaBP inside a cell affects the spatiotemporal aspects of Ca2+ transients depends on basic properties including the intracellular concentration, metal-binding affinities and kinetics and interactions with binding partners. The latter is typical for "sensor" CaBPs. Binding of Ca2+ induces a conformational change that allows the binding to specific targets, thereby modulating the function/activity of these targets. One of the aims of this project is the identification of putative targets for PV, CB and CR. From studies on different CaBP knockout mice, it is known that the absence of these CaBPs in distinct neuron subpopulation affects the properties of these neurons and also the properties of the neuronal networks in which these neurons operate. We expect that these changes are also manifest at the level of behavior. We will try to correlate our findings from mouse behavioral experiments with results obtained in humans, where in postmortem brains of patients with various neurological diseases, altered CaBP expression was observed. In our projects we will investigate whether these changes are causally linked to the disease or are the result of an adaptive/protective mechanism brought about by the Ca2+ homeostasome.

Direct link to Lay Summary

Last update: 21.02.2013

Communication	Title	Media	Place	Year

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Number	Title	Start	Funding scheme

Ca2+ signaling is a key mechanism for various biological processes such as cell growth, cell division, differentiation and cell death. It operates on a huge range of time scales (ms to h) and magnitudes (nM to mM). For this, cells make use of various proteins from the “Ca2+ signaling toolkit” comprising proteins involved in Ca2+ entry/extrusion (channels/pumps) from either the extracellular space or from intracellular compartments (endoplasmic reticulum, mitochondria) and specific Ca2+-binding proteins acting as intracellular modulators of Ca2+ signals. Each cell type expresses a particular set of the various components involved in Ca2+ handling/signaling defined as the “Ca2+ signalsome”; when considering that these molecules are not only functionally linked, but likely also regulated in a network-like fashion, the term “Ca2+ homeostasome” has been proposed. In view of the large complexity of Ca2+ signaling, it is not surprising that the malfunctioning or disruption of intracellular Ca2+ homeostasis has been associated with ageing, neurodegeneration and various diseases collectively termed “calciumopathies” that includes epileptic seizures, migraine, bipolar disorder and autism, to name a few. While genetic mutations in Ca2+ channels or Ca2+-regulated channels were identified in calciumopathy patients, none have been identified so far for the family of intracellular Ca2+-binding proteins including parvalbumin (PV), calbindin D-28k (CB) and calretinin (CR). However, altered expression (most often a down-regulation) has been observed in postmortem brains of patients with various diseases. Currently it is not known whether these changes are causally linked to the disease or the result of an adaptive/protective mechanism brought about by the Ca2+ homeostasome. Thus, the aim of this proposal is to better understand the web of intracellular Ca2+ signaling/regulation using transgenic mice with altered expression of these Ca2+-binding proteins.A) In this subproject we will investigate the putative Ca2+ sensor function of CR. The experiments are based on our findings that I) CR is able to interact with intracellular parts of specific Ca2+ channels in vitro and II) the presence of a putative LD domain, a protein-interacting domain present in several structural and functional proteins such as paxillin. The transient expression of CR during adult neurogenesis in the hippocampus linked to differentiation and expression of specific transcription factors (e.g. basic helix-loop-helix proteins) is another indication of CR’s putative Ca2+ sensor function.B) Initial experiments on the behavior of PV knockout mice indicate that PV’s absence could be causally related to one of the presumed calciumopathies: autism spectrum disorder (ASD), where altered Ca2+ signaling is suggested as a possible molecular mechanism. A detailed behavioral investigation of PV-/- mice is the core of this subproject.C) The functioning of the Ca2+ homeostasome is investigated at two levels: at the cellular level in a muscle cell line where PV levels are modulated by genetic methods and in muscle fibers from PV-/- and wildtype mice. We will determine the functional consequences of the observed increased mitochondria volume in PV-/- fast-twitch muscles with respect to Ca2+ handling, but also determine which Ca2+-dependent processes lead to these adaptive or homeostatic changes. At a systemic level, the role of the Ca2+-binding proteins PV, CB and calbindin D-9k in the fine-tuning of divalent cation (Ca2+, Mg2+) resorption and secretion in the kidney will be investigated by different methods including Gene Chip Analysis.In conclusion, the Ca2+-binding proteins PV, CB and CR likely play important roles in many processes in the brain, muscles and in the kidney and their absence or downregulation in humans and animal models leads to changes detectable at various levels. The projects of this proposal are not exclusively aimed to better understand the protein’s physiological roles, but to explore the regulation of the Ca2+ homeostasome, of which these proteins are an integral part. We think that this will eventually lead to a better understanding of various diseases (calciumopathies) and possibly ageing, in which alterations in Ca2+ signaling are thought to be causally involved.